Don Quixote is well-known for being an idealistic fool in a cold grey world, a laughingstock and Butt Monkey. As a result, there's a huge Misaimed Fandom that sees him as an ideal to strive for even if he can't win. The original Quixote was an idealistic fool, but it wasn't his entire character. Both the Misaimed Fandom and the people who laugh at him forget that he was also an unsympathetic snob, who used his "knighthood" as an excuse to not pay for things and to bully his social inferiors, especially Sancho. Part of why he isn't remembered this way is the Man of La Mancha, which emphasized his foolish idealism a lot more. ("Dream...the impossible dream...")
"No, senor, that's not true," said Sancho, "for I am more cleanly than greedy, and my master Don Quixote here knows well that we two are used to live for a week on a handful of acorns or nuts. To be sure, if it so happens that they offer me a heifer, I run with a halter; I mean, I eat what I'm given, and make use of opportunities as I find them; but whoever says that I'm an out-of-the-way eater or not cleanly, let me tell him that he is wrong; and I'd put it in a different way if I did not respect the honourable beards that are at the table."
Ginny Weasley has a reputation in the fandom for being a little tart who can't keep her knickers on. In the canon, she dated three guys (no, not at once) over a period of four years before marrying the last one. Interestingly, Hermione has also dated exactly three people, one for rather petty reasons, and no one accuses her of being a slut. * Note that some people claim that nobody (or hardly anybody) actually believes Ginny is a slut, and that anyone calling her a slut is merely doing so to piss off Ginny fans.
There's Dumbledore, who was gay. He did some other stuff, but mostly, he was gay. You can understand why Rowling didn't insert this fact into the books, since Dumbles' primary characteristic since the Word of Gay announcement is that he was gay. Man, people who refuse to read a sentence from the books or watch one minute of the movies are more likely to know that than any other detail of this importance inserted in the books!
Draco Malfoy is often called Ferret, Ferret Boy or some form of that insult in the fandom, referring to an incident in which Mad-eye Moody turns him into a bouncing ferret. In the book, there is not much mention of the incident after the fact.
There is one instance where Hagrid threatens to turn a misbehaving Malfoy into a ferret, although that's just one instance. But honestly, " Draco Malfoy, the Amazing Bouncing Ferret" is just too memetic.
Ron tends to be described by people as a slob and flat-out idiot, who gobbles food nonstop and is horribly abusive to his angelic wife, Hermione. In the books, Ron was more laid-back and certainly not as driven as Hermione, but it was mentioned that he and Harry did about equally well in their classes (Harry was mostly forced to learn extra things to save his own life). And Belligerent Sexual Tension aside, there was never any indication that Ron was or would be abusive towards Hermione, or indeed that he'd be capable of it if he wanted to be. The worst he ever did was when he abandons Hermione and Harry in Deathly Hallows, and that was done in a moment of anger, after enduring Mind Rape by the horcrux, and he wanted to return immediately, only to be foiled by circumstances. Oh, and his first act upon returning was to rescue Harry from drowning in a frozen pond. It gets so bad with him sometimes that he's become the Trope Namer for Ron the Death Eater.
James Potter won't live down being a jerk to Snape. People who've suffered bullying in the past would see James as a spoiled brat that got everything handed to him while Snape had to suffer his whole life.
Professor Slughorn to some degree. Many fans will still see him as just another example of a bigoted Slytherin, due to his surprise at a Muggle-Born being more skilled than a Pure-blood. This ignores the fact that he's genuinely appreciative of his favored students, especially Lily, and expresses genuine regret regarding telling Voldemort about how to make Horcruxes.
Then there's Professor Binns, the ghost who teaches History of Magic. His most memorable scene involved him droning about the Goblin Rebellion. Now, many HP fanfictions contain scenes that take place in class, and if it's History of Magic? You can bet Binns will be talking about goblins.
The books' publishing company, Bloomsbury, will always be the people who insisted the first book's title be changed to "Sorceror's Stone" in America, giving them a reputation of thinking Americans are a bunch of morons who would never pick up a book unless the title promised it was about magic.
Alanna from the Song of the Lioness suffers from a similar reputation, despite having had relationships with a grand total of three guys in her life, each of them long-term and serious, one of which became a marriage. Yet that's still more than any of Tamora Pierce's other protagonists (except Briar in the Circle of Magic universe), so she gets stuck with a Slutty McSluttington image in some readers' minds. (The books were banned from a few school libraries as a result of her supposed sexual promiscuity, which probably didn't help.)
The Immortals has one positive and one negative. The good: Daine and her rampaging zombie dinosaurs. The bad: hooking up with with Numair when he's 30 and she's 16. Although Alanna/George had an eight-year age gap and he was declaring his love for her while she was still a teenager, they were both still adults by modern standards, and it didn't push Student Teacher Romance squick buttons. The outcry was so great that Pierce promised never to have such a wide age gap again.
In-universe, Kel of Protector of the Small can't live down her fear of heights, and Wyldon takes every opportunity to test it. Works out for her in the end, though.
Pierce herself has this with the book Mastiff, which went over with her fans like a lead balloon and was largely seen as a shocking quality drop from a usually great author.
An in-universe example is Kyp Durron, a powerful young Jedi who once got either possessed or heavily influenced by an ancient and very evil ghost, and who then fished out an indestructible superweapon that had been dropped into the heart of a gas giant and proceeded to use it to cause a supernova that destroyed a rather populated planet. He was then very quickly and easily brought back into the light and put the superweapon into a black hole, then got off basically scot-free in the trilogy where he originally featured. Basically every book to feature him since then has called him on it, particularly I, Jedi, a sort of Fix Fic trying to get the trilogy to make sense, where the main character leaves in disgust after this mass-murderer is welcomed back into the Jedi Academy for training. Other books paint Kyp as the perpetual Atoner, having it and his lack of punishment constantly brought up.
Sometimes (including in I, Jedi) the death toll of Kyp's attack is vastly exaggerated, because apparently killing a few million people isn't bad enough; it's necessary to falsely claim he killed billions so that it can be put on the same level as the destruction of Alderaan (bonus fail points: said destruction is attributed to Darth Vader instead of the man who actually did it, Grand Moff Tarkin). The fact that the majority of the population were Imperial soldiers also tends to be ignored. Of course, Kyp himself doesn't consider that much of a comfort, given that many of those soldiers (including his own brother) were conscripts. And to make it worse, from New Jedi Order onward, Kyp is a Jerk Ass who's no longer interested in atoning for his sins.
A few of the people in The Bible have traits they'll never live down. "Doubting" Thomas springs to mind.
Bible researchers have also pointed out that the characters of those who told Thomas weren't impeccable either: John, who had an awful temper, and Peter, who denied Christ thrice.
Pontius Pilatus: washing his hands was basically a way of saying "I don't want to hear about this anymore". Now that's the only thing that whe heard about him, to an almont Trope Namer level.
This happens in continuity in the Wayside School book series. There were three kids in the class named Eric (Fry, Bacon, and Ovens). Eric Bacon and Eric Ovens were bad at sports so everyone just assumed Eric Fry was bad at sports when he was actually great. The only time people noticed him playing was when he caught a ball that slipped out of his hand. Everyone called him "Butterfingers" after that.
This happened with both of the others as well. Eric Bacon is much skinnier than the other two, yet he's called "Fatso;" Eric Ovens is easily the nicest and most easygoing, but he gets nicknamed "Crabapple."
In universe example with the Discworld character, Sam Vimes. One of his ancestors, Stoneface Vimes, led an army of rebels against the insane, murderous, pedophile king. But after their victory, no judge or jury could be found that would dare stand up against royalty. So Stoneface took matters into his own hands and performed the execution himself. Despite thus being one of the most important figures in Ankh-Morpork's history, the one thing everyone remembers about him is that he was a "regicide". The modern Vimes dislikes that term, saying "It was only one king. It's not like it was a habit."
It probably doesn't help that the modern Vimes reminds everyone of Stoneface.
And that he doesn't actually like the thought of kings (there's quite a passage about it in Men at Arms), at least, not the hereditary ones (the Low king of the Dwarves gets a pass for being elected and the position being non-hereditary).
Another in-universe example comes in Thud! when Vimes describes the painter Methodia Rascal as "Painted famous painting, thought he was a chicken, died."
Josella Playton in The Day of the Triffids writes a novel which her publisher ends up titling Sex Is My Adventure. Even years after civilization has collapsed and the eponymous killer plants are running amok, people she meets are still mentioning this book.
Richard Rahl, protagonist of the Sword of Truth, once led his troops to cut through a peace protest staged by Too Dumb to Livepolitical strawmen to get to the villains they're guarding, during the book that even fans acknowledge is best ignored. The key word here, of course, is "once". A good chunk of the times it's mentioned on this wiki, though, it's phrased to make it sound like he spends all eleven books doing nothing but slaughtering pacifists.
The infamous scene in which the demonic entities are first introduced... as a chicken.
"This looked like a chicken, like the rest of the Mud People's chickens. But this was no chicken. This was evil manifest."
In-Universe example in A Song of Ice and Fire: Jaime Lannister is forever known as the Kingslayer for murdering the man he was supposed to protect. What is forgotten is that the king in question was killed to prevent him from roasting a city full of people alive. The reason this became his defining moment was because only a few people knew that, and most of them were killed in the uprising going on that same night, or deliberately chose not to mention it.
Also in A Song of Ice and Fire, Catelyn Stark, despite being a kind and caring mother and a woman with a strong sense of honor and duty, is often remembered for being a stone-cold, callous bitch for her poor treatment of her husband's bastard son, Jon Snow: her son lies unconscious, forever a cripple, and in a moment of mad grief she tells the bastard, "it should have been you" before breaking down in tears. She never lives that one down, it seems. It's a big Kick the Dog moment, but let's be perfectly honest: who doesn't get one of those in this series?
It doesn't help that it's one of the first things we see her do, and to new readers struggling to get a hold of the series' Loads and Loads of Characters in their minds, something like that tends to stand out.
Twilight may be popular for other things, but now thanks to the movie, everyone associates the entire saga with sparklyvampires.
Baseball-playing sparkly vampires.
The sparkly vampires were already infamous (at least among Fan Haters) before the movie, but the movie definitely compounded it.
And of course, Jacob imprinting on Edward and Bella's newborn daughter, leading to many accusations that he "wants to fuck a baby" despite that not being quite what really happens (he waits until she's a physical adult before making a move). Though the whole subplot is still creepy enough that it even overshadows what would probably otherwise be a pretty infamous sequence in its own right where the superstrong unborn fetus breaks Bella's spine and Edward performs a C-section with his teeth, something that caused a genuine push to have the film adaptation be directed by David Cronenberg.
In Nursery Crime: The Big Over Easy by Jasper Fforde, Chief Inspector Spratt is constantly having to defend himself against a reputation for killing giants ("Technically, only one of them was a giant; the others were just tall.") Of course, his full name is Jack Spratt. As in 'Jack the Giant Killer', which might have something to do with it.
While Sherlock Holmes used cocaine in a more than a couple of stories, he was an addict, not of that drug, but of mental exercise. His brain had to be constantly working or else it would "rebel." When on a case or any other activity that he would consider mentally challenging, he would stop using for as long as his brain was entertained. If one only knew Holmes from fanfics and pastiches (professionally published fanfics), he turns into a drug-addicted genius who needs a constant high to function. In such stories, his drug of choice is usually changed to opium for whatever reason, even when it was specifically stated in the canon that he doesn't do opium. This is also carried over to more official adaptations.
Also the Breakout Villain Moriarty, who encountered Holmes once in person and also sent him a letter once, but is only ever remembered as 'Holmes's Arch-Enemy'.
In universe example in The Black Company series. In the first novel, Croaker writes romantic fiction about the Big Bad who is employing them. Almost two decades later, people are still bringing it up. This turns into a Crowning Moment of Funny in the third book when the same people find out that the person they thought was his girlfriend is the Lady incognito.
In the first Sweet Valley Twins book, Jessica shows up for ballet class decked out in sparkles and ribbons. The teacher proceeds to publicly humiliate her, blasting her for this, and for several months afterwards, acts completely oblivious to the fact that Jessica is the best dancer in the class, instead, blatantly favoring the less skilled Elizabeth.
In A Christmas Carol, Ebenezer Scrooge has become most famous for his 'catchphrase', "Bah! Humbug!" In reality, he uses this phrase only twice during the entire book.
In a both in and out of universe example, Isildur from The Lord of the Rings. He was a total Badass who did many great things. Chief among them, he actually defeated Sauron, admittedly with his father and Gil-Galad having done most of the work. However, all the Council of Elrond and many readers seem to remember him for is not destroying the ring, which meant Sauron could return. What makes this an especially glaring example is that no one could have done any better. Frodo is the next person to be in a position to destroy the Ring, and makes the exact same choice. According to Word of God, no one has enough willpower to actually destroy the ring.
In the movie version this is compounded, with Isildur only defeating Sauron with a desperate swing that luckily cut Sauron's ring finger off, and then refusing Elrond's urging to destroy the Ring. In the book there's no mention of Elrond taking Isildur to the Cracks of Doom, and it's questionable whether he ever said anything about destroying the Ring to Isildur.
In-universe example in the Southern Sisters Mysteries: when they were kids, Patricia Anne lost Mary Alice's Shirley Temple doll. They're in their sixties at the time the book takes place, and Mary Alice still brings it up a few times every book.
Harry Dresden has a few things he would rather people forget about, except they never do.
In-universe example: The Faerie Courts keep laughing about The Donut Incident.
After Changes, people keep bringing up the fact that he slept with Mab to become the Winter Knight.
Chances are, if you haven't read The Hitchhikers Guide To The Galaxy you're only going to know one thing about it: 42, the meaning of life. Which it isn't, by the way; it's the answer to the great Question of life, so it's utterly meaningless unless you know them both. Despite a generation of bored mathematicians and conspiracy theorists attempting to prove him wrong, the author insisted all his life that 42 was simply a number he grabbed out of the air and found amusing.
In-Universe in Vampire Academy. Adrian seems to get a lot of grief for drinking and smoking when he has an legitimate reason for doing so, rather than to annoy people.